Magnetic disk drives use servo patterns to encode information for positioning the read/write heads. These servo patterns are typically written as wedge-shaped sectors extending in the radial direction from the inner diameter of the disk to the outer diameter of the disk. Because the disk area reserved for servo information is area that cannot be used for storing user data, there is great value in making the servo area as small as possible. This invention addresses the common problem of how to encode and detect servo information (i.e., numerical servo data used for positioning) as well as customer data at the highest possible density with adequate signal-to-noise ratio (SNR).
One method for encoding numerical servo data is known as dibit encoding, in which a binary “1” is encoded as a pair of magnetic transitions while a binary “0” is encoded as the absence of any transitions. Dibit encoding, unfortunately, has two disadvantages related to the fact that only one symbol carries energy. First, because there is no timing content on the “0” symbol, the format typically requires the addition of “dummy dibits” to protect against long runs of zeros during which no timing recovery is possible, reducing the overall space efficiency of the pattern. Second, the SNR rendered by dibit encoding is suboptimal because the “0” symbol contains no energy. As understood herein, symbols which are very different from each other are more easily distinguished by the detector, but the “1” and “0” symbols for the dibit pattern are different for only half of the detection cell.
Accordingly, to overcome the above-noted disadvantages, data can be encoded using biphase encoding, in which a binary “1” is encoded as a pair of magnets, while a binary “0” is encoded as a pair of magnets of the opposite polarity. A matched filter can be used to determine correlation of the “1” and “0” symbols. It is to be appreciated that unlike dibit encoding, the biphase pattern carries energy in both symbols, resulting in a greater difference, i.e., detection distance, between the two symbols.
As critically recognized herein, however, not all of this detection distance can be realized with a simple matched filter and threshold approach because a sequence of symbols does not produce an isolated response. For some possible sequences of symbols, the detection distance is reduced significantly from that achieved by an isolated response. As further recognized herein, the detector generally is not a continuous time system, but rather is a sampled system to render advantages that are not easily implemented in a continuous time system, and unless the read back signal is sampled synchronously, additional variations will be present in the matched filter output due to variations in sampling phase. The result is that for some configurations of the biphase matched filter, the loss of detection distance for the worst case sequence compared to that achieved by an isolated response can be quite pronounced. Even if a Viterbi algorithmi is applied to the read back signal to resolve detector decisions and thereby improve performance, the present invention critically observes that to guarantee accurate sampling phase, a sampled system typically requires a longer synchronization field prior to the servo data fields, and that the synchronization field consumes valuable disk area. Furthermore, a synchronously sampled system must be properly equalized to accurately resolve detector decisions. To further complicate the system, the equalization must be dynamically adjusted as the linear density changes from the inner diameter to the outer diameter of the disk, resulting in a system which is difficult to setup, susceptible to error due to misequalization, and generally user unfriendly. Having made the above critical observations, the invention herein is provided.